1. Field of the Invention
The present invention generally relates to oversampled delta sigma (.DELTA.-.SIGMA.) modulation analog-to-digital (A/D) converters and, more particularly, to a digital filter for filtering and decimating the output signals of the .DELTA.-.SIGMA. converter.
2. General Description of the Prior Art
With recent advances in the use of digital signal processing techniques for analog signals, many refinements in the basic and well-known process of analog-to-digital (A/D) conversion have been required. A/D converters have become very sophisticated, providing conversions at high speeds and with increasing accuracy. High resolution A/D signal conversion can be achieved with lower resolution components through the use of oversampled interpolative (or delta sigma) modulation followed by digital low pass filtering and decimation. Oversampling refers to operation of the modulator at a rate many times greater than the Nyquist rate, whereas decimation refers to reduction of the clock rate down to the Nyquist rate.
Delta sigma modulators (sometimes referred to as sigma delta modulators) have been used in A/D converters for some time. Detailed general information can be obtained from the following technical articles which are hereby incorporated by reference:
1) "A Use of Limit Cycle Oscillators to Obtain Robust Analog to Digital Converters", J. C. Candy, IEEE Transactions on Communications, vol. COM-22, no. 3, March 1974, pp. 298-305. PA0 2) "Using Triangularly Weighted Interpolation to Get 13-Bit PCM from a Sigma-Delta Modulator", J. C. Candy et al., IEEE Transactions on Communications, vol. COM-24, no. 11, November 1976, pp. 1268-1275.
Broadly described, a delta sigma A/D converter uses an internal A/D converter of modest resolution and a complementary digital-to-analog (D/A) converter in a feedback loop. The feedback loop increases resolution of the A/D converter in a manner consistent with the high speed operation afforded by the internal A/D converter. In theory, any error in linearity caused by the D/A converter is effectively added to the input signal and appears at the output without attenuation. Hence, 1-bit A/D and D/A linear converters are commonly employed in the feedback loop. When multi-bit A/D and D/A converters are employed in the feedback loop, the overall circuit is generally referred to as an "oversampling" A/D converter.
The output signal of a delta sigma modulator is a bit-rate representation of the analog input signal. This bit-rate signal is decimated to a lower sampling rate and coded, usually in 2's complement format. The output signal of the modulator is filtered in order to remove high frequency quantization noise present in the single-bit representation of the analog input signal. Previously, the application of filter characteristics to the .DELTA.-.SIGMA. modulator output signal bit-rate was fixed by customizing the hardware used, depending on the desired filter shape. Manufacture of a general purpose delta sigma modulator has not been demonstrated.